1. Field of the Invention
The present invention relates to a resin material for denture base that is used mainly for relining (rebasing) of a denture base. Particularly, the invention relates to a two-liquid mixing type resin material for denture base, which is superior in operability and can be used by a simple operation of only mixing two liquids with each other during the use.
2. Description of the Conventional Art
Hitherto, the relining (or rebasing) of a denture base has been carried out in an indirect method. That is, a denture base in which one layer of a mucosal surface has been deleted is used as a tray; an impression material is built up thereon, thereby taking a functional impression; the resulting denture base is invested in a flask using an investment as it stands; the impression material is then eliminated, thereby forming a space in the investment; thereafter, according to the same procedures as in the preparation of a denture base, necessary amounts of a liquid and a powder for a resin material for denture base are weighed, and mixed and mixed by means of a spatula or the like; the mixed material in a dough-like state is filled in the space in the investment within the flask; the dough is heated for polymerization and curing as it stands; and after cooling the cured material is separated from the investment, followed by forming the surface characterization and polishing.
However, since it takes a long period of time to carry out the relining by this indirect method, a direct method using a low temperature polymerization type or photopolymerization type resin material for denture base is being frequently employed. The direct method is carried out in the following manner. That is, after deleting one layer in a mucosal surface side of a denture base, necessary amounts of a liquid and a powder for a resin material for denture base are weighed, and mixed with each other; the mixed material in a dough-like state is built up directly in the mucosal surface side of the denture base; the resulting material is inserted into an oral cavity of a patient, thereby repeating trials to impart a form; and then, the material is allowed to stand outside the oral cavity for polymerization or irradiated with light for polymerization, thereby completing the relining of the denture base.
The resin material for denture base that is used for such relining (rebasing) of a denture base has hitherto been constituted by a liquid made of methyl methacrylate as a major component and a powder made of polymethyl methacrylate or polyethyl methacrylate as a major component and having a polymerization catalyst added thereto. During the use, this resin material for denture base is required to be used under the conditions where necessary amounts of the liquid and the powder are weighed, uniformly mixed with each other, and allowed to stand for a certain period of time, whereby the mixture becomes into a dough-like state. However, according to this method, it is required that the powder and the liquid are weighed separately, and then mixed with each other, and hence, the operation is complicated; the mixed material in the dough-like state for a short time because the monomer has a high volatility; it is difficult to take a timing for building up on the mucosal surface of the denture base; and due to its odor or stimulation, an unpleasant feeling is imparted to an operator or a patient, and depending on circumstances, the health of the operator or patient is possibly injured.
In addition, with respect to the conventional art resin materials for denture base, though it is essential to mix the liquid and the powder with each other and to use the mixture, they involve such a defect that air is entrained during the mixing. The air entrained into the mixed material becomes air bubbles, leading to a reduction in physical properties such as strength and formation of minute irregularities on the denture base surface after curing, and causing staining or discoloration of the denture base or inducing an odor with a lapse of time. In recent years, in order to compensate these defects, a sheet-state photopolymerization type resin material for denture base that does not require mixing has been developed and provided. However, since the material before curing causes plastic deformation, this sheet-state photopolymerization type resin material for denture base involves such a defect that deformation is caused by pressure of a cheek mucosa, etc. or undercuts during the elimination from the oral cavity, thereby making it difficult to impress a precise state in the oral cavity to the material.
Thus, the present invention is aimed to provide a resin material for denture base that, even after mixing, is less in entrainment of air, is less in possibility to incorporate air bubbles into a mixed material, exhibits a proper elasticity during elimination from an oral cavity, not causing deformation, and can impress precisely the state in the oral cavity.
In order to achieve the above-described aim, we, the present inventors made extensive and intensive investigations. As a result, it has been found that by employing a two-pack mixing type liquid, the entrainment of air caused by mixing can be minimized and that it is useful to properly use monomers having a specific performance, leading to accomplishment of the invention.
In other words, it has been found the above-described aim can be achieved by a resin material for denture base comprising a combination of two liquids, in which one of the liquids is a solution of a polymer dissolved in a monomer having a high solubility to the polymer, and the other liquid is a solution of a polymer dispersed in a monomer that does not dissolve the polymer there in or having a low solubility to the polymer, leading to accomplishment of the present invention.
Specifically, the resin material for denture base according to the present invention is a two-pack type resin material for denture base constructed by the following components (a), (b) and (c):
(a) a methacrylate or acrylate monomer having at least one unsaturated double bond,
(b) one or two or more polymers selected from the group consisting of a methacrylate polymer, an acrylate polymer, a methacrylate/acrylate copolymer, a methacrylate/styrene copolymer, and an acrylate/styrene copolymer, and
(c) a polymerization initiator, wherein a liquid A comprises the polymer (b), the monomer (a) having a solubility to the polymer (b) of 20% by weight or more, and the polymerization initiator (c);
a liquid B comprises the polymer (b) and the monomer (a) having a solubility to the polymer (b) of less than 20% by weight; and when mixing the liquid A with the liquid B, the monomer (a) in the liquid A swells or dissolves the polymer (b) in the liquid B to increase its viscosity, whereby the mixture is polymerized and cured.
More specifically, the resin material for denture base according to the present invention is characterized in that two types of monomers having a different solubility to the polymer from each other are used and that a monomer having a high solubility to the polymer of 20% by weight or more is used in one of the liquids, while a monomer having a low solubility to the polymer of less than 20% by weight is used in the other liquid.
Concretely, the liquid A is one with a monomer having a high solubility to the polymer and having a viscosity adjusted by dissolving the polymer in the monomer. The liquid A has a performance such that when mixed with the liquid B later, its viscosity rapidly increases. Further, when the polymerization initiator is added to the liquid A, the liquid A will become polymerizable with an external energy such as a heat and light. On the other hand, the liquid B is one with a monomer that does not dissolve the polymer therein or has a low solubility to the polymer and has the like polymer as in the liquid A dispersed therein. As the polymer, are preferred polymers having a high solubility and a low glass transition point at normal temperature.
Such resin material for denture base comprising two liquids is extremely easy for mixing and is less in entrainment of air bubbles into a mixed material. In particular, when the resin material for denture base is filled in a cartridge where automatic mixing can be effected, the mixing can be carried out in a state free from air bubbles. When the polymer compounded in the liquid B is swollen by and dissolved in the monomer having a high solubility to the polymer to be compounded in the liquid A, the mixed material becomes into a dough-like state having a high viscosity and exhibits a proper elasticity. As a result, even when a few of undercuts or the like are present, the mixed material can impress precisely the shape in the oral cavity. Thereafter, the resulting mixed material is taken out from the oral cavity and irradiated with an external energy such as a heat and light to promote the polymerization of the monomer. Thus, it can function as a material for denture base.